1. Field of the Invention
The present invention relates to a device for wavelength coupling of laser beams with different wavelengths, comprising: at least one laser source for generating a plurality of laser beams, and at least one overlapping device for the spatial overlapping of the plurality of laser beams to form an overlapped laser beam with a plurality of wavelengths.
2. Description of Related Art
A device for the (dense) wavelength coupling of a plurality of laser beams comprises a laser source that generates a plurality of laser beams for overlapping or wavelength coupling. The laser beams are spatially overlapped in an overlapping device and form an overlapped laser beam that has a plurality of wavelengths. Spectrally sensitive elements, for example in the form of edge filters, may be used as an overlapping device. An angle-dispersive optical element is frequently used as an overlapping device. The laser beams striking at different angles are overlapped on the angle-dispersive optical element, because of their different wavelengths, to form a single laser beam with a plurality of different wavelengths. A reflecting or transmitting grating, which reflects or transmits the laser beams striking at different angles of incidence at a common emergent angle is frequently used as an angle-dispersive element. The gratings may, for example, be designed as dielectric reflection or transmission gratings, volume Bragg gratings (VBGs), or as gratings based on a hologram (volume holographic gratings), as described, for example, in EP2523280 A2. An angle-dispersive optical element, for example in the form of a prism may also be used as an overlapping device.
There are various possibilities for realising the angular distribution or generation of laser beams striking the angle-dispersive optical element at different angles. U.S. Pat. No. 6,192,062 B1 discloses, for example, a device in which an imaging optical element in the form of a lens is used. The lens converts a spatial distribution in a first focal plane, in which is arranged a plurality of amplifier elements (laser emitters), to an angular distribution in a second focal plane. The typically parallel laser beams emerging from the amplifier elements at different points in the first focal plane strike the diffraction grating at different angles as a result of the place-to-angle transformation, as required. The lens used for the transformation may be designed as a cylindrical lens or, as the case may be, a spherical lens, and is also referred to in the following as a transformation lens.
As an alternative to using an imaging optical element for realising the angular distribution on the angle-dispersive optical element, it is also possible to alter the direction of radiation of the laser beams emerging from a plurality of laser sources spaced apart from each other so that they strike the angle-dispersive optical element at different angles in an overlap region. To achieve this a plurality of lenses may be used for receiving and focussing the laser beams from the plurality of laser sources, the pitch between the lenses differing from the pitch between the laser sources, as described in detail in WO 2006/116477 A2.
To generate laser beams with different wavelengths for the overlap, wavelength stabilisation is required. To realise the stabilisation a feedback may be provided for each laser beam to be overlapped to stabilise the respective wavelength of the laser emitter assigned to the laser beam. In this case so-called volume Bragg gratings or grating waveguide mirrors that reflect some of the laser radiation back into the respective laser emitter can be used as feedback elements. The wavelength stabilisation may also be achieved by means of a common feedback element, for example by means of a so-called chirped volume Bragg grating that enables a plurality of laser emitters to be stabilised on different wavelengths. It is also possible to effect the feedback directly in a respective laser emitter, for example when using a so-called “Distributed Feedback” (DFB) laser, where the feedback element, in the form of a grating structure, is written into the laser-active medium itself. The feedback element or grating structure can also be arranged outside the laser-active zone, but in a waveguide integrated on the same chip, as is the case, for example, with the so-called “distributed Bragg reflector” (DBR) laser. The spectral bandwidth of an individual wavelength-stabilised laser beam is generally between approx. 0.1 nm and 0.4 nm in this case.
The feedback may also be achieved by means of a feedback element that is arranged in the path of the overlapped laser beam. In this case a partially reflecting output coupling element is frequently used as the feedback element, where the entire device, as far as the output coupling element, is used as a resonator (so-called “external cavity laser”). In such a device a beam telescope, consisting of two cylindrical lenses or two spherical lenses, can be used as disclosed, for example, in U.S. Pat. No. 8,049,966 B2. An individual lens can also be arranged in the path of the overlapped laser beam approximately at the distance of its focal length, as disclosed for example, in U.S. Pat. No. 6,192,062 B1.
To generate the laser beams the laser source typically has a plurality of laser emitters that are designed as laser diodes. A number of strip type laser diodes, typically between approx. 5 and approx. 55 or 60, may in this case be arranged next to each other on a common chip, which is also referred to as a laser bar. A laser diode typically generates a laser beam that diverges comparatively quickly in a first direction (FA (fast axis) direction), i.e. has a large beam angle of approx. 40° or more, for example, and diverges comparatively slowly in a second direction (SA (slow axis) direction), and propagates at a beam angle of approx. 15° or less, for example. The laser beam emitted from a respective laser diode has an almost diffraction-limited beam quality in the FA direction and has a comparatively poor beam quality in the SA direction, at least for currently known broad stripe emitters. The SA direction typically runs in the plane of the bar transverse to the stripe type laser diodes, and the FA direction runs perpendicular to it. The laser diodes can be arranged on the bar at a constant pitch to each other, but it is also possible for the pitch to vary between adjacent laser diodes of the bar. The beam parameter product of the individual laser diodes or stripe emitters of a bar in the SA direction may vary, as disclosed in EP 2 088 651 of the applicant, for example. To generate a variation of the beam parameter product at least some of the stripe emitters may be designed with different widths in the SA direction.
To collimate the exiting laser beams a cylindrical lens can be arranged on the bar or some distance from it to collimate the laser beams of the individual laser diodes in the FA direction. A collimation of the individual laser beams of the bar may also be conducted in the SA direction by means of a microoptic cylindrical lens array.
A plurality of laser bars that are arranged next to each other in the SA direction form a so-called horizontal stack. The diode bars can in this case be arranged on a common heat sink, for example by so-called “direct copper bonding”, DCB. If necessary cooling ducts can be installed in a DCB heat sink for water cooling of the diode bars. In this case the heat sink may be designed so that it is extremely thin, giving rise to a low pitch, i.e. a short distance between the individual laser emitters, since thermal crosstalk between the individual laser emitters is reduced by the low thermal resistance and the packing density can therefore be increased.
It goes without saying that the stacking of a plurality of laser bars one above the other (i.e. in the FA direction) is also possible, this being termed vertical stacking. Such a vertical stack may, in particular, be cooled on the back by a common DCB heat sink, allowing a high filling factor during stacking of the laser bars. A plurality of laser bars may also be arranged in an arc so that the emission directions are directed towards the inside of the arc, as disclosed in EP 1619765 A1, for example.
U.S. Pat. No. 8,049,966 B2 discloses a method of overlapping the laser radiation from a plurality of laser emitters or laser bars of a vertical stack arranged one above the other in the wavelength. In US 2011/0216417 A1 it is proposed to use a transformation device in the form of an optical rotator to rotate the laser beams of the diode bars or laser emitters of a horizontal stack by 90°, for example, thereby transposing the roles of the SA direction and FA direction before the laser beams are overlapped in the wavelength. In US 2011/0222574 A1 it is proposed to carry out a two-dimensional wavelength overlap of laser beams generated by a two-dimensional array of laser sources by means of two overlapping devices in the form of gratings.
In addition to the mechanical stacking described above, optical stacking of the laser beams generated by a plurality of laser emitters or laser bars is also possible. An interleaver is often used for this purpose. The interleaver may be used, for example, to interleave the laser beams of a vertical or horizontal stack of diode bars so that they lie one above the other after interleaving in one dimension or direction. The interleaving may for example, be achieved by the method disclosed in EP 1601072 A1. It is also possible to interleave the laser beams of two vertical stacks, for example, by means of slit mirrors as interleavers to reduce the filling factor or pitch between the laser beams generated by the individual diode bars and packing them optically tighter. It is also possible to displace, relative to each other, the laser beams of a horizontal stack of diode bars arranged next to each other in the SA direction by means of a first stack of plates or mirrors rotated into a fan shape in the FA direction and to generate a stack of laser beams in the FA direction by means of a further stack of plates or mirrors rotated against each other, which stack may be arranged rotated 90° to the first stack, for example, as disclosed in DE 10 2011 016 253 A1.
In addition to diode bars each of which have a plurality of laser emitters, so-called single emitters are also commonly used, i.e. laser diodes where only one single emitter is arranged on a chip or bar. The collimation lenses in such a single emitter in the FA and SA directions are often also arranged on the heat sink of the chip or adjacent to it. A stack can also be formed from single emitters. In conventional stacks a plurality of single emitters can be arranged adjacent to each other on different stages of a stepped base body (heat sink). The parallel running laser beams generated by the single emitters may, for example, be arranged above each other, e.g. via a mirror device in one dimension or direction in space.
In the wavelength overlapping of laser beams generated by a plurality of laser bars a comparatively high laser power can be achieved. In such an overlap the emitters or laser diodes of a laser bar may have the same wavelength. For example, U.S. Pat. No. 8,049,966 B2 discloses a method of stacking a plurality of horizontal stacks of laser bars one above the other and overlapping the laser beams generated by bars arranged one above the other in the wavelength. However, the beam quality in such an overlap is normally inferior to that when the laser beams of a plurality of single emitters are overlapped, each of which has a different wavelength. Such an overlap of the laser beams generated by a plurality of laser emitters arranged on a common bar may be achieved, for example, by arranging for the alignment of the laser beams to be rotated by means of a transformation device before the overlap, as disclosed, for example, in US 2011/0216417 A1. Although in this case a high beam quality can be achieved after the overlap, the total power that can be achieved for an individual external resonator in a wavelength overlap of the laser beams of a plurality of single emitters is poorer, comparatively speaking.
In both cases the laser beam generated in the overlap can be supplied to a beam guide system for further use. The overlapped laser beam is frequently coupled into an optical fibre. Power scaling is possible before coupling by means of spatial multiplexing or polarisation multiplexing. Further coarse spectral coupling may also be carried out by means of dielectric edge filters, as described in detail in U.S. Pat. No. 8,049,966 B2 for example. To ensure efficient coupling of the overlapped laser beam into the optical fibre the beam profile of the overlapped laser beam should be adapted to the cross-section of the optical fibre. The adaptation of the beam profile may be performed by means of suitable beam telescopes, for example. Typically symmetrisation of the beam quality, which generally differs in two directions perpendicular to each other after overlapping, is also required. For this purpose a beam transformation of the overlapped laser beam can be carried out to generate an essentially symmetrical laser beam for the coupling. For adaptation of the beam profile, i.e. the distribution of intensity of the laser beam transverse to the direction of propagation, multiplexing by spatial juxtaposition of a plurality of overlapped laser beams outcoupled from a plurality of external resonators, for example, is also possible, cf. for example EP 2 088 651 A1. Adaptation of the beam profile or beam quality by polarisation coupling can also be carried out.
The overlapped laser beam or beams may be coupled into a beam guide, for example in the form of an optical fibre, and can be used for pumping a laser, for example a solid state laser. A laser beam generated by the solid state laser or the laser beam generated by the wavelength coupling device can itself be used for the treatment of materials, particularly for laser cutting, laser welding, laser drilling, hardening, re-melting, powder application, etc.